This invention relates to an optical device for evaluating the contours of a workpiece and particularly to one which uses moire principles and enabling phase shifting or field shifting analysis.
Optical contouring systems using moire principles have been in use for some time. In one type of such system, a known periodic pattern such as a grating is projected onto the surface to be evaluated and the image of the grating as taken from a direction askew from the illumination direction as deformed by the surface is analyzed to determine the profile. The deformed grating image is often super-imposed upon a matched reference grating which amplifies the grating deformation and results in the well-known moire fringe patterns. Such fringe patterns are easily interpreted as surface contours by an observer or through automated means. The resulting fringe patterns are lines of equal depth change, which are independent of object orientation, rigid body displacement, color or marking on the part. The typical approach for providing the moire pattern is to use separate projection and reference gratings which are positioned in the optical paths of the illumination and viewing system, respectively. Similarly, separate primary lenses are used for projecting the focused image of the grating onto the object and for focusing that image onto an image detector or eyepiece.
Although present moire contouring systems operate satisfactorily, the use of separate gratings and primary lenses produces a number of disadvantages. For example, slight differences in the magnification of two gratings on the object and detector produce changes in the moire interference pattern which are independent of workpiece contour. When such magnification errors exist, the differences produce a fringe pattern due to a displacement of portions of the projected and reference gratings, even when the workpiece is truly flat. Similarly, slight differences in focus and/or alignment of the two primary lenses cause distortion of the image. The use of separate primary lenses and gratings also produces a stability problem, since even minute relative motion between these elements can cause image distortion. Even when purchasing expensive optics, it is difficult to obtain two lenses which are preciously matched in terms of their magnification and focal length. In addition to the above mentioned disadvantages, the requirement of separate lens and grating elements further increases the cost of the imaging device since these elements must be separately purchased, mounted, installed and aligned.
The present invention is related to a moire contouring imaging device or camera which overcomes the above mentioned disadvantages of prior art designs. The advantages of the present invention are achieved primarily through the use of a single optical grating and primary lens for both the projection and viewing optical paths for improved stability and optical performance. The optical system in accordance with this invention features the illumination beam light path being inclined with respect to the viewing light path, both of which pass through a common grating and primary lens. Due to the inclination of the two beams, one of the beams must be redirected so that they will converge upon a focal plane where the object to be evaluated is placed. In accordance with the embodiment of this invention shown in the appended drawings, the illumination optical path is reflected by a folding mirror to cause it to converge with the viewing optical path upon the object being evaluated. The invention further incorporated features to enhance its simplicity, durability and definition of produced images by reducing the problems of stray light within the device.
The moire contouring imaging device of this invention is also readily adapted for providing either phase shift or field shift type moire analysis. If the grating for the illumination and viewing optical paths of the device of this invention is shifted, a phase shift is produced in which the moire image is presented at multiple phases of position across the viewing field. The effect is to cause the moire interference pattern to scan across the field of view. Field shift analysis is provided through providing a translation mechanism which physically moves the grating.
The moire imaging device of this invention is also readily adapted for use in providing field shift moire analysis. This effect is caused by translating the folding mirror perpendicular to the mirror surface through the use of a linear translation device. In field shifting, both the lens and projection grating appear to be moved together and thus the projection point of either the illumination beam path or viewing beam path changes. Like phase shifting, field shifting provides the potential of providing absolute measurement of surface contour. In assignees co-pending application Ser. No. 564,104 filed on Aug. 8, 1990 entitled "Field Shift Moire System", now U.S. Pat. No. 5,069,549 a process for field shift moire evaluation is described. This related application is hereby incorporated by reference.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.